Semiconductor light emitting devices have been known, which are fabricated by stacking, on the entire surface of a sapphire substrate, an n-side contact layer made from GaN doped with Si, an n-side cladding layer made from GaN doped with Si, an active layer made from InGaN doped with Si, a p-side cladding layer made from AlGaN doped with Mg, and a p-side contact layer made from GaN doped with Mg. These devices having such a structure have been commercially available as industrialized blue and green LEDs (Light Emitting Diodes) for emission of light in a range of 450 nm to 530 nm.
Various methods have been proposed to increase brightness of semiconductor light emitting devices. These methods are mainly classified into a method of improving a luminous efficiency on the basis of a current inputted in the device and a method of improving a light emergence efficiency by allowing emitted light to efficiently emerge out of the device. The former improvement of the luminous efficiency is dependent on a material forming a crystal layer, a crystal structure, a crystal growing ability, a combination of crystal layers, and a fabrication process. With respect to the latter improvement of the light emergence efficiency, it is required to examine reflection of light depending on a device structure and an array structure of the devices mounted on a system device, and it is important to allow emitted light to emerge out of the device without damping and leakage.
With respect to the method of improving the light emergence efficiency on the basis of an input current, particularly, in a device having a tilt crystal plane tilted from the principal plane of a substrate, a light emitting region composed of a first conductive type layer, an active layer, and a second conductive type layer can be formed on the whole or part of the S-plane. In the case where the device is formed into an approximately hexagonal truncated shape, a first conductive type layer, an active layer, and a second conductive type layer can be formed even on the upper surface parallel to the principal plane of a substrate. In the case of a semiconductor light emitting device formed into a flat shape having a plane parallel to the principal plane of a substrate, light is damped by multi-reflection. On the contrary, in the case of the semiconductor light emitting device having the S-plane, since light emission is performed by making use of the S-plane, light is allowed to emerge out of the device from the light emergence plane without the effect of multi-reflection. The crystal layer having the S-plane may be configured such that part of the crystal plane forming the S-plane function as the first conductive type layer. Further, the crystal layer having a crystal plane not perpendicular to the principal plane of a substrate is effective to improve the light emergence efficiency.
In a semiconductor light emitting device having a tilt crystal plane, the luminous efficiency can be enhanced by making use of good crystallinity of the tilt crystal plane. In particular, in the case of injecting a current only in the S-plane having good crystallinity, since the S-plane exhibits good incorporation of In and good crystallinity, the luminous efficiency can be enhanced. In addition, the area of the active layer extending within a plane substantially parallel to the S-plane can be made larger than the area of the active layer projected on a substrate or the principal plane of an underlying growth layer. With this configuration, since the area of the light emission area becomes substantially large, it is possible to reduce a current density, and to reduce saturated brightness and hence to increase the luminous efficiency.
By the way, in the above-described semiconductor light emitting device having the tilt crystal plane tilted from the principal plane of a substrate, if the device has a hexagonal pyramid shaped crystal layer, the state of steps of a portion, neat the vertex, of the S-plane becomes poor, with a result that the luminous efficiency of the vertex portion is degraded. The reason for this is as follows: namely, assuming that a plane of the hexagonal pyramid shape is divided into a vertex side area, a left side area, a right side area, and a bottom side area by two lines passing through with respect to an approximately center portion of the plane, the state of steps on the vertex side area is particularly wavy, with a result that abnormal growth of crystal is liable to occur in the vertex side area. On the contrary, in each of the right and left side areas, steps densely extend in the forms of approximately straight lines, with a result that crystal is very desirably grown, and in the bottom side area, steps are slightly wavy, with a result that the crystal growth state is poorer than that in each of the right and left side areas.
If a p-side electrode is formed in a region, in which the state of steps is undesirable, of the tilt crystal plane, the luminous efficiency on the basis of an input current becomes lower than that of the case where the p-side electrode is formed in a region in which the state of steps is desirable. Accordingly, it is desirable to form an electrode in a region other than an area in the vicinity of the vertex portion in which the state of steps is wavy and an area in the vicinity of the bottom surface in which the state of steps is slightly wavy.
On the other hand, in the case of forming an electrode on a tilt crystal plane, since the electrode is tilted relative to a light emergence plane of the device, it is possible to suppress multi-reflection of emitted light and hence to suppress damping of light, and to allow light reflected from the electrode to emerge out of the light emergence plane provided on the bottom surface of the device.
However, if an electrode is not formed in the areas, in the vicinities of the vertex and the bottom surface of the device, of the tilt crystal plane, the ratio of a light component not reflected from the electrode in the light emergence direction to light emitted by a light emitting region becomes high.
Further, of the light reflected from the electrode formed on the tilt crystal plane, a light component having a large incident angle relative to the light emergence plane is totally reflected from the light emergence plane and thereby is not allowed to emerge out of the device. As a result, even if an electrode is formed only in a good crystalline region to improve the luminous efficiency, it is impossible to enhance the light emergence efficiency and hence to sufficiently obtain the effect of forming the light emitting region only in a good crystalline area.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a semiconductor light emitting device capable of enhancing the luminous efficiency while increasing the light emergence efficiency, and a fabrication method thereof, and to provide an image display system and an illuminating system, each of which includes an array of the semiconductor light emitting devices, and fabrication methods thereof.